Head closure system

ABSTRACT

In order to reduce the time required for refueling a nuclear reactor, the studs utilized for securing the head of the reactor pressure vessel are provided with modified breechblock threads at their lower ends, and also at their upper ends. After being detensioned, the studs are rotated 60* for disengagement from corresponding threads in a flange at the top of the pressure vessel and are lifted out of the stud holes when the head is removed. The studs are tensioned and detensioned by a plurality of portable hydraulic stud tensioning devices or tensioners which are attached to the upper ends of the studs by the modified breechblock threads at the upper ends of the studs. The tensioners are carried by a separate hoist system mounted on the closure head for the vessel.

ilnite States Patent Friseii et a1.

[ Dec. 3, 1 974 1 1 HEAD CLOSURE SYSTEM [731 Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

[22] Filed: Sept. 13, 1971 {21] Appl. No.: 179,703

[52] U.S. Cl 292/256.73, 176/87, 292/251,

220/55 D [51] int. Cl. E05c 5/04- [58] Field of Search 52/224; 85/1 L, l T, 42;

176/87; 220/3, 44, 55 D, 46 R, 46 MS; 292/251, 256, 256.67, 256.73

3,707,107 12/1972 Bieri ..85/1T Primary Examiner-Richard E. Moore Attorney, Agent, or Firm-J. R. Campbell 5 7 ABSTRACT In order to reduce the time required for refueling a nuclear reactor, the studs utilized for securing the head of the reactor pressure vessel are provided with modified breechblock threads at their, lower ends, and also at their upper ends. After being detensioned, the studs are rotated 60 for disengagement from corresponding threads in a flange at the top of the pressure vessel and are lifted out of the stud holes when the head is removed. The stiids are tensioned and detensioned by a plurality of portable hydraulic stud tensioning devices or tensioners which are attached to the upper ends of the studs by the modified breechbloek threads at the upper ends of the studs. The tensioners are carried by a separate hoist system mounted on the closure head for the vessel.

21 Claims, 10 Drawing Figures PATENIEL DEC 3 I974 sum 1 or 4 IHHYI-FII'IIIHHHHIP PATENTELUEB 3W 3,851,906

SHEEI 20F 4 FIG. 2A

PATENIEL 9E3 sum 3 or 4 .l .l l llv ll l l l ll ll ll l l H & m l 'M" l ll ll lndl n 'l lll l PM'ENTEL BEE I 3 4 saw u 0F 4 A-STUD CROSS SECTION AREA Y-STUD ELONGATION PER PITCH WITH FULL LOAD APPLIED 8 P-STUD THREAD PTCH HEAD CLOSURE SYSTEM CROSS REFERENCE TO RELATED APPLICATIONS This application relates to an application, Ser. No. 179,645 entitled Head Closure Mechanism" filed Sept. 31, l97l by Erling Frisch, Harry N. Andrews and P. B. Haga and assigned to the Westinghouse Electric Corporation.

BACKGROUND OF THE INVENTION This invention relates, generally, to pressure vessels and, more particularly, to systems for securing and releasing the closure heads of nuclear reactor vessels.

Heretofore, the closure heads of large nuclear power reactors have been secured to the pressure vessel by a large number of studs which are threaded into the vessel flange and extend through corresponding holes in the head flange. The necessary loading on the studs has been achieved by hand tightening of the upper nuts while the studs are being preloaded by a small number, usually three, of portable hydraulic tensioning devices which are applied to the studs in a prearranged sequence until all studs are equally loaded.

In order to remove the closure head for refueling, the studs must be tensioned until the nuts become unloaded and can be backed off by hand. Following this, all studs are completely removed and stored before the closure head is lifted off and refueling operations can commence. On a typical reactor, the studs are seven inches in diameter and there are eight threads per inch covering nine inches of stud length. Therefore, approximately 140 complete revolutions of each stud are required for removal and subsequent replacement.

Also, each of the portable tensioners is connected to a stud by means of a six-inch long nut, forming part of the tensioner actuator shaft, to an upper threaded extension of the stud. Accordingly, approximately 100 revolutions of the shaft are required for each application of the tensioner and two applications per stud are normally required to obtain the proper stud loading.

Considering that a typical four-loop reactor has 56 studs, each weighing 600 pounds, and that great care must be taken during the turning to prevent galling of the threads. it is realized that considerable time is required to remove the closure head after shutdown and to replace it after refueling. Actually, the time required for each ofthese operations averages three days if there are no complications. In order to make possible more rapid refueling of a nuclear reactor, it is necessary to reduce this time.

SUMMARY OF THE INVENTION In accordance with one embodiment of the invention, the necessary reduction in the time required to remove and to replace the closure head by an operating crew is achieved by replacing the prior studs with a smaller number, for example 36, of larger diameter studs and by utilizing a modified breechblock system for attaching the studs to the pressure vessel flange as well as for connecting them to portable hydraulic tensioners. The tensioners are carried by a separate hoist system mounted on a shroud structure encircling the reactor control rod drive mechanisms. The shroud structure is mounted on top of the closure head. The engagement or disengagement of the studs to either the pressure vessel or to the tensioners is accomplished by a rotation of the studs or the tensioner shafts, requiring only a relatively short time on the part of the operating crew. After all studs have been detensioned or unloaded and rotated to the clear position, they are simultaneously lifted out of the stud holes when the closure head is removed. The studs remain in place on the closure head during the refueling operation, thereby saving the time previously required for separate removal and storage. Because of the relatively large clearances provided between adjoining modified breechblock threads when the studs are detensioned, there is a minimum possibility of galling of the threads.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the nature of the invention, reference may be had to the following detailed description, taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a view, in elevation, of a head closure system embodying principal features of the invention;

FIGS. 2a and 2b, when joined end-to-end, constitute a sectional view through a portion of a reactor vessel and closure head and one of the holding studs and a portable tensioning device;

FIG. 3 is a view, in plan, of the structure shown in FIG. 2a..

FIG. 4 is a view, in section, taken along the line IVIV of FIG. 2b.,

FIG. 5 is an enlarged view, in section, taken along the line V-V in FIG. 2b.,

FIG. 6 is an enlarged detail view taken along the line VI-VI in FIG. 1, partly in section and partly in side elevation, of a hoist for one of the portable tensioning devices;

FIG. 7 is a view, in front elevation, of the hoist shown in FIG. 6;

FIG.8 is a view, in section, of a portion of a modified holding stud; and

FIG. 9 is a view, in section, taken along the line IX-IX in FIG. 8.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawings, particularly to FIGS. 2A and 2B, the structure shown therein comprises a portion of a generally cylindrical reactor vessel 10 having an integral outwardly extending flange 11, and a closure head 12 having an integral outwardly extending flange 13 which mates with the flange-ll on the vessel 10. The vessel 10 may be of a type, well known in the art, suitable for use in a nuclear power system.

As shown more clearly in FIG. ll, the head 12 is retained on the vessel 10 by means o'fa plurality, for example 36, of holding studs 14. As shown in FIG. 2B the studs 14 are disposed in vertically aligned holes 16a and 16b in the head flange l3 and the vessel flange 11,

respectively. The hole 16a extends through the head 1 flange 13 and the hole 16 terminates in the vessel flange lll. As shown in FIG. 1, the reactor is in operating condition with all studs 14 in position on the flange 13 of the head 12 with tension applied. The necessary tensioning of the studs is produced by a small number, for example three, of portable hydraulic tensioning devices 22 which are applied to the studs in a prearranged sequence.

As explained hereinbefore. it is desirable to reduce the operating time required to remove and to replace the closure head during a refueling operation. In accordance with the present invention, the necessary reduction in operating time is achieved by utilizing a modificd breeehblock system for attaching the holding studs to'the pressure vessel flange as well as for connecting them to the hydraulic tensioning devices or tensioners. Engagement or disengagement of the studs to either the pressure vessel or to a tensioner is accomplished by a 60 rotation of the studs or the tensioner shaft.

As shown in FIGS. 2A and 28, each stud 14 has a plurality of separate parallel horizontally extending interrupted threads 17a at its lower portion and similar threads 17b at its upper portion. Each hole in the vessel flange has threads 18a therein corresponding to the threads at the lower portion of the stud. Likewise, threads 18b, corresponding to the threads 17b in the upper portion of the stud, are provided in a generally cylindrical extension 19 on an actuator shaft 21 of the hydraulic tensioner 22 which will be described more fully hereinafter.

It should be noted that the interrupted threads 17a and 17b at the lower end and the upper end, respectivcly, of the stud actually are generated by cutting separate parallel grooves in the stud rather than a continuous helical groove, which is the case with standard breechblock threads. This modification is utilized to prevent any contact with the corresponding threads when rotating the stud or the tensioner actuator shaft to the lock" position.

Because of the 'extreme difficulty in machining threads of this type, to the required tolerances, directly in the vessel flange, the threads 18a are cut in a separate bushing 23 which, in turn, is attached to the flange 11 by standard or Acme threads 24 which may be machined in the flange without difficulty. The completely finished bushing 23 is threaded into the flange to the proper depth and is locked in place by a pin 25 so that the interrupted portions of the threads assume the same position on all bushings relative to the vessel centerline.

The threads 18b may be machined directly in the ex-' tension 19 which is threaded onto the lower end of the actuator shaft 21 and secured thereto by a pin 26.

The use of the bushings 23 hasthe additional advantage of permitting selection of a material better suited for the application than the vessel material. The bushings may also, if desired, be plated with a corrosion resistant material, such as nickel or zinc, to reduce the corrosive effect of water when the vessel is submerged during the refueling period. It also makes it possible to replace a bushing in case of accidental damage to the main threads.

As shown more clearly in FIG. 5, in which the stud is shown in the clear" position, the threads on the stud and in the vessel flange or bushing are aligned in vertical sections with gaps between sections having a greater angular extent than the angular extent of the thread sections. In the present structure the threads are arranged in three vertical sections. Thus, after the studs have been inserted in the bushings, they must be rotated 60 until thread sections 17a and 18a overlap as indicated in FIG. 5.

' In order to avoid any change of interference between the threadsduring therotation, relatively large clearances are provided between the horizontal surfaces of adjoining threads. As shown in FIG. 4, each stud 14 may be rotated by means of a straight cylindrical handle 27 projecting radially from the stud and attached thereto by threading into the stud. Two stop pins 28, attached by threading into the underside of the head flange 13, as shown in FIG. 2B, limit the rotation in either direction. A hinged locking bar 29 prevents accidental movement of the studfrom either position. A support 31 for the locking bar 29 is attached to the flange 13 by screws 32. In order to avoid interference with normal rotation of the stud, the locking bar 29 may be swung out of the way to a position indicated by dot-dash lines in FIG. 2B.

As explained in the aforesaid copending application, stud rotation is facilitated by reducing the frictional forces to a minimum. For this purpose, a thrust ball bearing 33 is attached to the lower end of each stud. The bearing 33, shown more clearly in FIG. 8, is attached to the stud by means of a clamp plate 34 and a bolt 35. A circular plate 36 having a large central hole serves as a seat for the thrust bearing when the stud is in the inserted position. The plate 36 is attached to the bushing 23 by means of several bolts 37. The vertical distance from the seat 38 of the support plate to the bushing threads is so selected as to obtain maximum axial clearance between stud and bushing threads in this position. The stud is centered in the bushing by a taper 39 in the support plate hole.

As explained hereinbefore, the necessary tensioning of the studs is achieved by utilizing a small number, for example three, of the portable hydraulic tensioning devices 22 which are applied to the studs in a prearranged sequence. When in position for service, as illustrated in FIG. 2A, the tensioner 22 rests on the flat upper surface of the head flange l3 and is rotationally aligned with the stud by an alignment pin 41. In general, the tensioner 22 comprises two pistons 42 disposed in two cylinders 43 supported between a base plate 44 and a top plate 45 which are held together by four tie rods 46 as shown in FIG. 3. The pistons are mounted in tandem to obtain the required stud loading without exceeding a hydraulic pressure of 10,000 psi. A heavy cylindrical bushing 47 is attached to the base plate 44, as by welding, and serves as the support for the tensioner when it rests on the head flange 13. The bushing 47 encloses the upper portion of the stud 14.

The tensioner actuator shaft 21 is surrounded by the pistons 42. As explained hereinbefore, the cylindrical extension 19 is fixed on the lower end of the shaft 21. The extension 19 contains the threads 18b corresponding to the threads 17b on the upper portion of the stud 14. Sufficient clearances are provided between the horizontal surfaces of the threads 17b and 18b to prevent interference during subsequent rotation of the extension 19 to the engaged or lock position. A thrust ball bearing 48, similar to the bearing 33, is attached to the lower end of the actuator shaft 21. When the tensioner is positioned, the outer bearing race 49 rests on the upper flat surface of the stud l4 and the entire weight of the actuator shaft assembly is carried by the bearing. The bearing location is selected to obtain maximum axial clearance between threads 17b and 18b. The bearing also reduces the effort required to rotate the actuator shaft to the engaged or lock" position bymeans of a hand wheel 51. The correct amount of rotation in either direction is insured by a pointer 52, fixed on the actuator shaft, and two stop pins 53 located on the top plate 45.

After the actuator shaft assembly is in the lock position, the tensioner is gradually pressurized by supplying hydraulic fluid underneath the pistons 4-2 through openings 54 in the cylinders 13. The developed upward thrust is transmitted through a spherical washer 55 and a fixed member or nut 56 on the actuator shaft assembly which moves freely upwardly until contact is established between threads 17b and 18b. As shown, the nut 56 is threaded on the upper portion of the shaft 21 and is fixed by a set screw 57. Continued movement causes lifting of the main stud 14 until stud threads 17a contact bushing threads 18a. Further application of pressure results in tensioning of the stud until the desired loading is obtained. At this point, the pressure is maintained at a constant value while an adjustable member or main nut 58, which is threaded on the stud 14, is tightened by hand until firm contact is established with a spherical washer 59 and the head flange 13. When the hydraulic pressure is reduced, the stud load is transferred to the adjustable member 58 and the tensioner pistons are returned to the lower position by application of hydraulic pressure above the upper piston through an opening 61 in the upper cylinder 43.

The rotation of the adjustable member or main nut 58 is accomplished in this example by a hand wheel 62 which drives a gear 63. The gear 63 causes rotation of a thin-walled tube 64 which is constructed to permit a small vertical movement. Teeth 65, provided in a thickened portion at the lower end of the tube 64, are slidably disposed in slots 66 provided in the upper outer surface of the main nut 58 to rotate the nut.

The actual value of the stud loading may be estimated by a micrometer 67 on top of the actuator shaft which indirectly measures the elongation or strain of the stressed part of the stud by comparing its length with that of an unstressed rod 68 of the same material located in a central hole in the stud. Since this method is well known in the art, no further explanation of its operation will be furnished herein.

After a stud has been properly tensioned and secured by the main nut 58, the hand wheel 51 is rotated to the clear position to disengage the threads 17b and 18b.

The tensioner 22 may now be lifted straight up by means of lifting lug 69 and hoist hook 70 and transferred to another stud.

In preparation for removing the closure head 12 for refueling. the tensioners 22 are again positioned on all studs in a prearranged sequence and the following procedures executed to unload the studs. The hand wheel 51 is moved to the lock position and hydraulic pressure is applied-under the pistons thereby stretching the stud 14 until the main nut 58 is completely unloaded. Next, the nut is backed off a short distance, the piston pressure is removed, and the pistons are returned to the lower position. This causes return of the main stud 14 and the actuator shaft assembly to the lowest position, as illustrated on the drawing. Following this, the actuator shaft may be rotated to the clear position and the tensioner removed. After all studs have been unloaded in this manner. they are rotated to the clear" position by means of the handles 27 to disengage the stud threads, and the headmay be lifted with all studs in place in the head.

The upper, external part of a reactor with the herein described Head Closure System is shown in FIG. 1. As illustrated, the reactor is in operating condition with all studs 14 in position on the flange 13 ofthe head 12 with tension applied. Control rod drive mechanisms 71, extending from the top of the head to a missile shield 72 are encircled by a generally circular shroud structure 73 which may be made of angle irons 74 and steel plates 75. For convenience, the shroud structure may be made in three sections and it is secured to the head 12. The missile shield 72 is attached directly to the head 12 by several heavy rods 76. Ventilating holes 77 are provided near the bottom of the shroud for a natural circulation cooling of the mechanism coils. The wiring for the control rod mechanisms and also for nuclear instrumentation is brought through multi-conductor cables 78 to a cable tray 79 which .is attached by hinge joints to the shroud structure 73. g

The cable tray, which is at least 7 feet wide in a typical structure, makes it impossible to service six or more of the head closure studs, directly below the tray, by 1 any overhead crane. For this reason, and for convenience, a separate hoist system is provided below the cable tray 79. A hoist mechanism 81 is provided for each of the portable stud tensioners 22. The hoist mechanisms travel on a circular track 82 which is supported on brackets 83 attached to the angle irons 74 of the shroud structure. The track diameter is such that the hoist hooks are directly on the stud circle. Thus, the stud tensioners may be easily and rapidly lowered into position on any of the closure studs by means of the hoist mechanisms 81.

In order to avoid interference between the interrupted threads at the upper part of the studs and on the tensioner actuator shaft extension, the tensioner must be accurately aligned rotationally during the lowering operation. This is achieved by providing alignment guides 84 which are attached to the shroud structure 73. An alignment guide 84 is provided for each stud 14. The guides 84 enter a slot 85, cut in the tensioner base plate 44, as indicated in FIG. 3. If desired, the tensioners may be removed from the reactor during normal operation by moving the hoists with the withdrawn tensioners to a position on the track 82 accessible to an overhead crane sling 86. This permits removal of hoist and tensioner as a unit without requiring any transfer of load. The removal of a stud may be accomplished in the same manner.

The details of a suitable hoist mechanism 81 are shown in FIGS. 6 and 7. Each hoist mechanism comprises a trolley carriage 87 having two heavily flanged trolley wheels 88 which ride on top of the circular track 82. The track 82 includes an I-beam. 89 having a rail 91 secured to the top flange of the beam. The I-beam 89 is attached to the brackets 83 by gusset plates 93. Motive power is provided by an electric gear motor 92 connected to one of the trolley wheels 88.

In order to prevent accidental derailing of the trolley carriage 87, a safety latch 94 is provided to limit any upward movement to a safe value. The safety latch 94 includes a shaft 95 rotatably mounted on the trolley carriage 87 and a roller 96 rotatably mounted on a pin attached to the end of a lever arm 97, secured to the shaft 95. In the latched position, the roller is brought in close proximity to the bottom flange of the I-beam I 89. When it is desired to remove the hoist from the In order to reduce the threaded length of each stud to a minimum, it is important that the total load on a stud be distributed equally on all threads. The modified stud structure shown in FIGS. 8 and 9 meets this requirement. The stud member 14A is shown lifted from its bottom position until the lower thread 17a of the stud is in contact with the lower thread 18a of the bushing of holding member 23. However, no actual load is applied on the stud. lt will be noted that a gradually increasing clearance of Y to 5Y is provided between successive mating threads, where Y represents the elongation per pitch under maximum stud load. In effect, this means that bushing thread pitch is (P Y) as compared to P for the stud thread pitch. In order to make this possible, while assuming equally distributed load on all threads, it is necessary that the stresses (psi) resulting from the maximum stud loading be the same over the whole length of the threaded portion of the stud.

As illustrated, this is achieved by machining a centrally located axially extending hole 106 with varying diameters in steps in the lower portion of the stud 14a so that the remaining stud area is proportional to the load carried by any particular section of the stud. As shown, the diameters become smaller in steps until the upper stud thread is reached. For a six-thread stud the lower section. which carries only one-sixth of the total load, will have across-sectional area of one-sixth of the maximum stud area. The next section will have twosixths of the maximum area and so on until maximum area is reached at the top of the hole which is substantially coextensive with the threaded portion of the stud. Thus, the number of stud sections corresponds to the number of threads on the stud. I

If load is gradually applied to the stud, the lower stud section will lengthen until the gap lY is closed. At this time, the load on the lower thread is one-sixth of the total load and will not increase beyond this value. Fur- .ther increase in the stud load will cause closing of gap 2Y and so forth until allgaps are closed when full stud loading is reached with all thread loadings being equal.

Because the corresponding sections of the bushing 23 have a much larger area than the lower stud sections, it will have a negligible effect on the value of Y. Also, the same holds true for the upper section wherethe load is transferred to the pressure vessel flange 11 through threads 24. The stud hole 106 is covered at the lower end by a cap 107 attached to the stud by bolts 108. The thrust ball bearing 33 is centered on a hub of the cap and is secured by the clamp 34 and bolt 35. The central rod 68, which is used in the measurement of stud loading, now continues through the threaded section and rests on the upper surface of the cap. This is permissible since the elongation per inch of this section now is the same as that of the stud itself. The plate 36, which serves as a seat for the thrust bearing when the stud is in the bottom position, is attached to the bushing 23 by bolts 37 as previously described. The modified stud structure may also be used at the upper stud end for connection to the hydraulic tensioner.

From the foregoing description, it is apparent that the invention provides a head closure system which makes it possible to remove and to replace the closure head of a nuclear reactor pressure vessel in relatively short periods of time with a relatively small amount of equipment. Therefore, the reactor refueling time and the cost of the equipment are materially reduced by utilizing the present system in which provision is made for quickly attaching portable tensioners to holding studs and detaching the tensioners from the studs.

We claim: 7

1. A head closure system for a generally cylindrical pressure vessel and a closure head having outwardly extending mating flanges with a plurality of vertically aligned holes therein, comprising studs rotatably disposed in said holes to retain the head on the vessel, each stud having a plurality of separate parallel horizontally extending interrupted threads at its lower portion and also at its upper portion, each hole in the vessel flange having threads therein corresponding to the threads at the lower portion of each stud, said threads being aligned in vertical sections with gaps between sections having a greater angular extent than the angular extent of the thread sections, said stud being rotatable to align the thread sections on the studs with the thread sections in the vessel flange, portable tensioning means adapted for mounting on said closure head for applying tension on each stud to retain the head on the vessel, and said tensioning means having threads therein corresponding to the threads at the upper portion of the stud for attaching the tensioning means to the stud.

2. The head closure system defined in claim 1, in-

cluding a rotatable member mounted in the tensioning means and containing the threads corresponding to the threads at the upper portion of the stud; and a device on said tensioning means for turning said rotatable member to selectivelyalign the threads thereon with the stud upper portion threads to thereby permit said tensioning means to selectively elongate said stud.

3. The head closure system defined ,in claim 2,

wherein the rotatable member comprises a vertically disposed shaft, and including thrust bearing means at the lower end of the shaft supporting the shaft on the upper end of the stud.

4. The head closure system defined in claim 3, including an extension on the shaft extending below said bearing means and containing the threads corresponding to the threads at the upper portion of the stud.

5. The head closure system defined in claim 2, including additional means for rotating said stud through a predetermined number of degrees separately from the rotatable member in the tensioning means.

6. The head closure system defined in claim 5, including locking means for preventing rotation of the stud during rotation of the rotatable member in the tensioning means.

7. The head closure system defined in claim 4, wherein the tensioning means includes at least one hydraulically actuated piston surrounding the shaft.

8. The head closure system defined in claim 7, including a member fixed'on the shaft for transmitting thrust from the piston to the shaft, and an adjustable member on the stud below the extension on the shaft to which the stud loading is transferred when the hydraulic pressure is removed.

9. The head closure system defined in claim 8, including mechanical means for actuating the adjustable member.

10. In a head closure system for a generally cylindrical pressure vessel and a closure head having outwardly extending mating flanges with a plurality of vertically aligned holes therein, in combination, holding studs disposed in said holes to retain the head on the vessel, each stud having a plurality of separate parallel horizontally extending interrupted threads at its upper portion, portable tensioning means for applying tension on each stud to retain the head on the vessel, said tensioning means having threads therein corresponding to the threads at the upper portion of the stud for attaching the tensioning means to the stud, said threads being aligned in vertical sections with gaps between sections having a greater angular extent than the angular extent of the thread sections, a rotatable member mounted in the tensioning means and containing the threads corresponding to the threads at the upper portion of the stud, means for rotating said member to align the thread sections in the member with the thread sections on the stud, a generally cylindrical shroud structure mounted on the head, a circular track supported by the shroud structure, and at least one hoist mechanism movably mounted on the track for handling the portable tensioning meansv 11. The combination defined in claim 10, including alignment guides attached to the shroud structure for aligning the tensioning means with reference to the studs.

12. The combination defined in claim 10, wherein the circular track includes an l-beam having a rail secured to its top flange, the hoist mechanism comprises a trolley carriage having a flanged roller riding on the rail, and safety latch means disposed in close proximity to the bottom flange of the l-beam to prevent derailment of the trolley carriage.

13. The combination defined in claim 12, wherein the latch means includes a shaft rotatably mounted on the trolley carriage and means operable by the shaft into close proximity to the bottom flange of the I-beam.

14. The combination defined in claim 13, including a handle for rotating the latch shaft, and a spring-biased latch pin for retaining the handle in preselected positions.

15. A tension load distribution system, comprising a holding member having a hole therein for receiving a tension stud member, said stud member having a plurality of separate parallel horizontally extending interrupted threads thereon, said holding member having threads in the hole corresponding to the threads on the stud member, said threads being aligned in vertical sections with gaps between sections having a greater angular extent than the angular extent of the thread sections, one of said members being rotatable to align the thread sections on the stud member with the thread sections in the holding member, stud tensioning means mounted on said holding member for elongating the stud member when the stud member and holding member threads are in alignment, and said stud member having an axially extending hole therein with varying diameters which respectively correspond with said threads mounted on the stud member, thereby to permit the application of equal loading on all stud members threads when the stud member is elongated by said stud elongation means.

16. The tension load distribution system defined in claim 15, wherein the hole is substantially coextensive with the threads on the stud.

17. The tension load distribution system defined in claim 15, wherein the diameters become smaller in steps extending from the entrance to the hole.

18. The tension load distribution system defined in claim 17, wherein the decreasing hole diameter steps provide corresponding stud sections having increasing cross-sectional areas.

19. The tension load distribution system defined in claim 18, wherein the number of stud sections corresponds to the number of threads on the stud.

20. The tension load distribution system defined in claim 15, wherein the holding member thread pitch is the quantity (P Y) where P is the stud member thread pitch and Y is the elongation per pitch under maximum stud load.

21. The tension load distribution system defined in claim 20, wherein the loading on all threads is substantially equal when maximum load is applied on the stud. l 1 =l 

1. A head closure system for a generally cylindrical pressure vessel and a closure head having outwardly extending mating flanges with a plurality of vertically aligned holes therein, comprising studs rotatably disposed in said holes to retain the head on the vessel, each stud having a plurality of separate parallel horizontally extending interrupted threads at its lower portion and also at its upper portion, each hole in the vessel flange having threads therein corresponding to the threads at the lower portion of each stud, said threads being aligned in vertical sections with gaps between sections having a greater angular extent than the angular extent of the thread sections, said stud being rotatable to align the thread sections on the studs with the thread sections in the vessel flange, portable tensioning means adapted for mounting on said closure head for applying tension on each stud to retain the head on the vessel, and said tensioning means having threads therein corresponding to the threads at the upper portion of the stud for attaching the tensioning means to the stud.
 2. The head closure system defined in claim 1, including a rotatable member mounted in the tensioning means and containing the threads corresponding to the threads at the upper portion of the stud; and a device on said tensioning means for turning said rotatable member to selectively align the threads thereon with the stud upper portion threads to thereby permit said tensioning means to selectively elongate said stud.
 3. The head closure system defined in claim 2, WHEREIN the rotatable member comprises a vertically disposed shaft, and including thrust bearing means at the lower end of the shaft supporting the shaft on the upper end of the stud.
 4. The head closure system defined in claim 3, including an extension on the shaft extending below said bearing means and containing the threads corresponding to the threads at the upper portion of the stud.
 5. The head closure system defined in claim 2, including additional means for rotating said stud through a predetermined number of degrees separately from the rotatable member in the tensioning means.
 6. The head closure system defined in claim 5, including locking means for preventing rotation of the stud during rotation of the rotatable member in the tensioning means.
 7. The head closure system defined in claim 4, wherein the tensioning means includes at least one hydraulically actuated piston surrounding the shaft.
 8. The head closure system defined in claim 7, including a member fixed on the shaft for transmitting thrust from the piston to the shaft, and an adjustable member on the stud below the extension on the shaft to which the stud loading is transferred when the hydraulic pressure is removed.
 9. The head closure system defined in claim 8, including mechanical means for actuating the adjustable member.
 10. In a head closure system for a generally cylindrical pressure vessel and a closure head having outwardly extending mating flanges with a plurality of vertically aligned holes therein, in combination, holding studs disposed in said holes to retain the head on the vessel, each stud having a plurality of separate parallel horizontally extending interrupted threads at its upper portion, portable tensioning means for applying tension on each stud to retain the head on the vessel, said tensioning means having threads therein corresponding to the threads at the upper portion of the stud for attaching the tensioning means to the stud, said threads being aligned in vertical sections with gaps between sections having a greater angular extent than the angular extent of the thread sections, a rotatable member mounted in the tensioning means and containing the threads corresponding to the threads at the upper portion of the stud, means for rotating said member to align the thread sections in the member with the thread sections on the stud, a generally cylindrical shroud structure mounted on the head, a circular track supported by the shroud structure, and at least one hoist mechanism movably mounted on the track for handling the portable tensioning means.
 11. The combination defined in claim 10, including alignment guides attached to the shroud structure for aligning the tensioning means with reference to the studs.
 12. The combination defined in claim 10, wherein the circular track includes an I-beam having a rail secured to its top flange, the hoist mechanism comprises a trolley carriage having a flanged roller riding on the rail, and safety latch means disposed in close proximity to the bottom flange of the I-beam to prevent derailment of the trolley carriage.
 13. The combination defined in claim 12, wherein the latch means includes a shaft rotatably mounted on the trolley carriage and means operable by the shaft into close proximity to the bottom flange of the I-beam.
 14. The combination defined in claim 13, including a handle for rotating the latch shaft, and a spring-biased latch pin for retaining the handle in preselected positions.
 15. A tension load distribution system, comprising a holding member having a hole therein for receiving a tension stud member, said stud member having a plurality of separate parallel horizontally extending interrupted threads thereon, said holding member having threads in the hole corresponding to the threads on the stud member, said threads being aligned in vertical sections with gaps between sections having a greater angular extent than the angular extent of the thread sections, one of said members being rotatable to aLign the thread sections on the stud member with the thread sections in the holding member, stud tensioning means mounted on said holding member for elongating the stud member when the stud member and holding member threads are in alignment, and said stud member having an axially extending hole therein with varying diameters which respectively correspond with said threads mounted on the stud member, thereby to permit the application of equal loading on all stud members threads when the stud member is elongated by said stud elongation means.
 16. The tension load distribution system defined in claim 15, wherein the hole is substantially coextensive with the threads on the stud.
 17. The tension load distribution system defined in claim 15, wherein the diameters become smaller in steps extending from the entrance to the hole.
 18. The tension load distribution system defined in claim 17, wherein the decreasing hole diameter steps provide corresponding stud sections having increasing cross-sectional areas.
 19. The tension load distribution system defined in claim 18, wherein the number of stud sections corresponds to the number of threads on the stud.
 20. The tension load distribution system defined in claim 15, wherein the holding member thread pitch is the quantity (P + Y) where P is the stud member thread pitch and Y is the elongation per pitch under maximum stud load.
 21. The tension load distribution system defined in claim 20, wherein the loading on all threads is substantially equal when maximum load is applied on the stud. 